Electroless nickel/gold plating liquids have been used in order to improve the corrosion resistance of a base metal, solder bondability, and wire bondability. In particular, these plating liquids are used on package boards that contain semiconductor bumps or semiconductors, and boards for mobile phones and the like.
In recent years, there have been demands to further improve the corrosion resistance and solder bondability in order to improve the reliability in conjunction with increased performance of electrical equipment, and inserting electroless palladium plating between electroless nickel plating and electroless gold plating has been effective. The electroless palladium plating liquid can be the plating liquids shown in Patent document 1 or Patent document 2. These electroless palladium plating liquids contain a sulfur compound as a bath stabilizer, and therefore trace amounts of sulfur are codeposited in the palladium film, but the sulfur will segregate, which causes a loss in corrosion resistance and solder bondability.
Patent Document 1: U.S. Pat. No. 3,204,035
Patent Document 2: Japanese Patent Publication H8-28561